Amorphous alloys lack long periodicity because of the irregular orientation of their metal atoms, and they are structurally unique in comparison with crystalline alloys, in that they have no crystal grain boundary or lattice defects. Because of this, amorphous alloys have excellent magnetic properties. They are particularly promising in applications as low hysteresis loss materials and high permeable magnetic materials. For example, Fe-based amorphous alloys have a high saturation density, and their application has been considered in iron cores for transmitters, making use of their properties of low hysteresis loss. They are said to have a much reduced loss, and thus lower costs, in comparison with conventional silicon steel lamination. Also, Co-based amorphous alloys have a lower magnetic coercive force over a wide frequency range, and are used as magnetic cores for magnetic amplifiers and the like.
In the case of magnetic recording, high density recording is requirement for 8 mm VTRs (video tape recorders), high vision VTRs, etc. In order to achieve such high density recording, it is necessary not only to improve the magnetic tape, but also the magnetic head. The magnetic head material must have a high magnetic flux density at high frequencies. For this reason, various constructions of magnetic heads have been devised. For example, in MIG (metal in gap) type magnetic heads, the portion near the magnetic gap of a ferrite yoke is sharpened for concentration of the magnetic flux on the recording/reproducing surface, and a magnetic layer with a high magnetic permeability and a high magnetic flux density is provided on the tip thereof, to thus improve the sensitivity of the recording and reproduction. The use of amorphous alloys in such magnetic layers promises advantages for the reasons mentioned above.
The most commonly used method for producing amorphous alloys is quenching. This is a method of forming an amorphous alloy by feeding molten metal to a cooled revolving roller and then rapidly cooling it by 10.sup.5 -10.sup.6 deg/s to harden it without allowing time for crystallization. However, amorphous alloys produced by quenching presently have a minimum thickness of a few dozen .mu.m or more. This is because irregularities occur on the surface of the molten metal when it is contacted with the cooling roller, making it difficult to form a thin film. The shape thereof is tape-like or filamentous. When used in a magnetic head or the like, it must be cut off and pasted onto the magnetic head yoke, but this involves a number of technical difficulties, including not only the strength and durability with the adhesive surface, but also the magnetic gap produced by the thickness and adhesive surface of the amorphous alloy. For this reason, amorphous alloys made by quenching is not applied to magnetic heads.
Other methods of producing amorphous alloys which have been studied include the sputtering, vacuum deposition and ion plating methods. Their application to magnetic heads has also been considered. However, the slow deposition rate of these methods renders their productivity poor and one must use special equipment which requires a large investment for facilities. These methods prevent low-cost mass production.
On the other hand, methods of producing amorphous alloys using electrolytic plating or non-electrolytic plating have also been studied (U.S. Pat. 4,101,389, Japanese Unexamined Patent Application SHO 55-164092, European Patent Specification NO. 422,760).
Plating methods make it possible to form amorphous alloys with satisfactory adherence to the magnetic head yoke. In addition, the equipment is simpler and productivity is higher, making these methods more suitable for the production of magnetic heads.
Nevertheless, in plating methods there are a limited number of metalloid elements which allow a stable amorphous state. Plating methods require salts of metalloid elements that are soluble in solvents (generally water). The metalloid elements include silicon, carbon, boron and phosphorus, but there are no appropriate water-soluble salts containing salts of silicon and carbon, and although water-soluble salts of boron exist, their oxidation-reduction potential is very undesirable compared to that of the transition elements such as iron, cobalt and the like, and thus it is difficult to obtain a eutectic mixture thereof.
For such reasons, phosphorus is usually used as the metalloid element in plating methods, as may be seen in the above-mentioned European Patent Specification No. 422,760 and U.S. Pat. No. 4,101,389. The donor salt which provides the phosphorus for the amorphous alloy may be phosphorous acid or a salt thereof, or hypophosphorous acid or a salt thereof, and they may easily form alloys with the transition metals.
FIG. 1 shows the crystallization temperature of phosphorus-containing amorphous alloys plotted against their content ratios. In the case of alloys with low phosphorus contents, two thermal peaks due to crystallization are observed, while in the case of those with high phosphorus contents there is only a single one near 350.degree. C. However, the crystallization temperature does not rise even if the phosphorus content is further increased.
Incidentally, in the case of magnetic heads as mentioned above, glass is fused in the gap of the sharpened recording/reproduction surface of the ferrite yoke. The glass used is one with a low melting point, but still temperatures of at least 450.degree. C. are needed. If phosphorus is used as the metalloid element for plating, then the crystallization temperature of the alloy is as low as 350.degree. C., and thus at 450.degree. C. or greater crystallization of the alloy occurs, thus causing deterioration of the magnetic properties of magnetic head. While plating offers excellent industrial advantages, problems still remain in the production of magnetic heads.
On the other hand, amorphous alloys produced by methods other than plating contain large amounts of the metalloids silicon, boron, phosphorus and carbon (normally 20 atomic or more), for elevation of the crystallization temperature. Alternatively, 4d metals or 5d metals are used instead of metalloids. However, these methods tend to reduce in the magnetic property of saturation flux density, and thus a large amount of the amorphous material becomes necessary.
Furthermore, when amorphous alloys are used in electronic devices they usually are required to have a high degree of reliability with long-term use. Amorphous alloys are said to be thermodynamically metastable, and are sometimes considered to have doubtful durability. Considering this, while the amorphous alloys produced by plating are not problematic within the temperature range of normal use, from the point of view of the above-mentioned durability, they presently do not offer satisfactory reliability.